The invention relates to the manufacture of containers, in the course of which their internal wall is coated with a layer comprising a barrier material.
It is known to deposit a barrier material by plasma. This technology is well explained in European Patent EP 1068032 in the name of the applicant, or else in U.S. Pat. No. 5,522,351.
A device used for such deposition is also described in the abovementioned document EP 1068032. This device comprises an electromagnetic wave generator, a cavity connected to the generator and made of a conductive material (generally metal), along with a chamber positioned in the cavity and made of a material (generally quartz) that is transparent to the electromagnetic waves coming from the generator.
After the container has been introduced into the chamber, a moderate vacuum (around 30 mbar to 100 mbar) is created in the chamber, while a high vacuum (a few μbar) is created in the container. A precursor gas (such as acetylene) is introduced into the container, this precursor being activated by electromagnetic bombardment (generally with low-power UHF microwaves at 2.45 GHz) to transform it into the cold plasma state and so to generate species, including hydrocarbons (comprising CH, CH2, CH3) which is deposited as a thin film (around 60 nm to 200 nm) on the internal wall of the container.
The container to be treated is generally made of a thermoplastic polymer such as PET (polyethylene terephthalate) which is, at 20° C., transparent to electromagnetic microwaves. The same applies to quartz, the dielectric properties of which, being favorable (at a temperature of around 20° C.) to microwave transmission, make it particularly attractive for the production of the chamber surrounding the container.
In laboratory conditions, the treatment of a container, including the application of the material then its preparation (in particular regarding temperature and pressure), may take several minutes or even several hours.
For industrial production, a series of devices of the abovementioned type (for example, around twenty) are mounted on a carousel and operate continuously for the scheduled treatment of several thousands of containers. The unit cycle time (that is, per device) is several seconds.
At this rate, problems arise with which laboratory workers are not generally confronted. Thus the inventors have noticed, in the course of the industrial process, the appearance of irregularities in the thickness of the barrier layer and of deformations of the PET containers.